Technical Field

The invention relates generally to circuits for detecting communication line conditions and, more particularly, to a circuit for detecting passive (on-hook) and active (off-hook) states of the lines. Background Art

In telephone systems, trunk circuits connect transmission lines to telephone offices. The receiving portion of a trunk circuit includes the power supply for the transmission line and a circuit for detecting high (on-hook) and low (off-hook) impedance levels of the line. A distant office may request use of a transmission line by causing it to assume the off-hook state. When the line is off-hook, the power supply causes sufficient current flow in the line to operate the detector circuit. After the initial off-hook state has been detected, the detector may be utilized to detect routing and other supervisory signals on the line.

Generally, power supplies, such as battery circuits, are designed for a maximum length line. For short lines, the standard battery circuit will tend to deliver an excessive amount of current. Elimination of high current consumption in short lines is desirable not only to protect the line apparatus but also to allow a reduction in power dissipation requirements of the batteries and to reduce the generation of heat and the overall current consumption of the telephone office. Power supplies which provide a constant current to the line independent of the line length are well-known in the prior art. Since the impedance of the line varies directly with the length of the line, a constant current battery circuit will supply a wide range of voltages. Operating within this wide range of voltages, detector circuits in current use have been able to detect state

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transitions of both long and short lines where the difference between the minimum on-hook (high) impedance of a short line and the maximum off-hook (low) impedance of a long line is large, such as, e.g., 24,000 ohms. This situation is commonly found in lines where the on-hook state presents an open circuit condition. However, in continuously terminated lines where the impedance difference may be minimal, e.g., 400 ohms, such detector circuits cannot detect the difference in state between an on-hook short line and an off-hook long line. Disclosure of Invention

A circuit in accordance with this invention detects both on-hook and off hook states of a communication line independently of the length of the line. The detector circuit comprises a current sensor and a voltage sensor, which are coupled to the line and respond to the current in and the voltage on the line, respectively, for generating two reference signals. A comparison of these two reference signals is used to generate two output signals. One output signal represents the on-hook state of the line when the magnitude of one of the reference signals is greater than the magnitude of the other. The other output signal represents the off— hook state of the line when the magnitude of the one reference signal is less than the magnitude of the other reference signal. A resistor in a line electrically isolated from ground and two resistors serially connected across the line sense the current in and the voltage on the line, respecti ely, to generate two reference potentials. A comparison of these two potentials generates output signals representing the on-hook and off- hook states of the line. In addition, the comparison output signals may drive, an opto-isolator to generate corresponding output signals electrically isolated from the comparison output signals. Brief Description of Drawings

FIG. 1 is a representation of a telephone communication system in general block diagram form;

FIG. 2 shows a trunk circuit for use in a telephone communication system in accordance with the invention;

FIG. 3 shows a floating battery feed circuit for 5 use with a trunk circuit in accordance with the invention; and

FIG. 4 shows in greater detail the detector circuit for use with a trunk circuit in accordance with one specific embodiment of the invention. 10 Best Mode of Carrying Out the Invention

A plurality of telephones 110 are connected to a telephone switching office 100 by subscriber lines 115. Office 100 may also be connected to one or more distant offices by transmission lines 165. A switching office may 15 comprise a network 130, line circuits 120, service circuits 150, trunk circuits 160, and _a controller 140. Line circuits 120 provide an interface between subscriber lines 115 and network 130. Service circuits 150 include such circuits as, signal pulse receivers, tone circuits, 20 etc. Trunk circuits 160 interface the network 130 and transmission lines 165. The controller 140 senses the operational state and activities of the line, service, and trunk circuits to detect certain signaling information, controls the network to establish connections between the 25 various circuits, and manages the state of the circuits as requi red.

The details of trunk circuit 160 are depicted in FIG. 2. One side of trunk circuit 160 is connected to network 130 via leads 201, and the other side is connected 30. to transmission lines 165. The network leads and the transmission lines are magnetically coupled and electrically isolated by transformer 211. The transformer also electrically isolates the transmission lines from ground to minimize longitudinal currents which are 5 frequently induced therein by electrical power lines.

Primary winding 202 of the transformer is connected across leads 201 through DC blocking capacitor 216, isolating the

transformer from direct current flowing from battery line feed and supervision circuit 210; secondary winding 203 is connected across lines 165 via DC blocking capacitor 212, isolating the transformer from direct current flowing from floating battery feed 215.

When the trunk circuit 160 is connected to a subscriber line 115 through network 130, circuit 210 supplies power to the subscriber line and supervises its status, reporting on-hook and off-hook states. In addition, the power supplied from circuit 210 may be electrically isolated from ground to reduce power consumption and longitudinal currents if the subscriber line has also been likewise isolated.

Detector circuit 214 detects the on-hook and off-hook states of transmission lines 165. Floating battery feed 215 supplies power to lines 165, and inductors 213 and 217 with capacitor 218 help prevent voice signal loss on the transmission lines. Detector circuit output conductors 221 are coupled by capacitor 218 and connected to transmission lines 165 through inductors 213 and 217. Floating battery feed 215 supplies power to the transmission lines through detector circuit 214 via leads 220. Floating battery feed circuit 215, FIG. 3, comprises transformer 311 having primary coil 312, secondary coil 313, and sense windings 314 and 315. Secondary coil 313 is connected to conductors 220 via diode 330 and LC filter circuit 341. Current flow in primary coil 312 is controlled by means of transistor 320. Base current is supplied to transistor 320 from terminal VI via transistor 360, resistor 362 and conductor 321 or from sense winding 315 via resistor 326. Transistor 320 may be inhibited by clamping conductor 321 to ground by means of transistor 361 or comparator circuit 338. The base drive to transistor 361 is cut off by inverter 369. Detector circuit 214, Fig. 4, comprises four major components: a voltage sensor, a current sensor, a comparator, and an opto-isolator. The current sensor,

resistor 410, is connected in series with the line 165 and generates a reference signal or potential representing the current in the line. Connected in series across the line, resistors 415 and 416 act as a voltage sensor, generating a second reference signal or potential representing the voltage applied by battery feed 215 to the line via conductors 220 and varying from 30 to 110 volts. Emitter- connected, switching transistors 420 and 421 and current- limiting resistors 412, 413, and 414 are connected to function as a comparator circuit, comparing the reference signals from the current sensor and the voltage sensor.

When the signal from the voltage sensor is greater than the signal from the current sensor, the comparator output signal, taken across collector resistor 412 will assume a level indicative of the on-hook state or high impedance level of transmission line 165. The output signal could also be taken across collector resistor 414, although at a level different from that across resistor 412. Transistors 420 and 421 will be in OFF and ON states, respectively, the voltage across emitter resistor 413 being developed by the current flowing from transistor 421. I t is assumed that the gain of transistor 421 is such that only a minimum amount of current is supplied through the base. The value of resistor 414 must be such that transistor 421 will not saturate with the highest voltage applied to the base while the power dissipated by transistor 421 is kept to a minimum. With transistor 420 OFF, no current flows through collector resistor 412 which is connected to one of conductors 221.

When the signal from the voltage sensor is less than the signal from the current sensor, the output signal will assume a level indicative of the off-hook state or low impedance level of transmission line 165. Transistors 420 and 421 will be in ON and OFF states, respectively, the voltage across emitter resistor 413 being developed by the current flowing from transistor 420. The voltage drop, for

example, 2 or 3 volts, across resistor 411, which is connected between collector resistor 412 and the base of transistor 420, must be enough to forward bias light- emitting diode (LED) 430. Collector resistor 412 causes several hundred microamps to flow through transistor 420 before any current flows through LED 430; thus, sensitivity to variations in opto-isolator 431 gain can be minimized. The value for resistor 413 must be small enough to ensure that opto-isolator 413 will operate in an off-hook state but large enough to minimize error due to differences between the base-emitter voltage drop of transistors 420 and 421.

Opto-isolator 431, which comprises LED 430 optically coupled to photo trans is tor 422, and electrically isolates the comparator output signals from the detector output signals, as well as ground. The collector of phototrans is tor 422 is connected to positive potential source 418 via current limiting resistor 417 and to detector circuit output terminal 419, the emitter being connected to ground. When the line is in the off-hook state, the detector output signal on terminal 419 assumes a first level. Current flowing through LED 430 causes the LED to generate an optical signal to phototrans istor 422. Phototrans is tor 422 conducts current, causing terminal 419 to assume the first level. The detector output signal may then be used to drive one of any number of well-known scanning devices for signaling the off-hook status of the line to controller 140. Conversely, when the line is in the on-hook state, the detector output signal assumes a second level with no current flowing through LED 430 and photo trans is tor 422.

The detector circuit can also be described in terms of a well-known four-legged impedance, bridge circuit with the impedance of the transmission line and resistor 411 constituting the unknown leg and resistors 410, 415, and 416 with known impedances, each constituting one of the other three legs. The bridge

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circuit comprises four impedance legs and, as previously described, a comparator circuit and an opto-isolator for electrically isolating the output signals.

The three known legs of the bridge define the unknown when the bridge is at a nullity, that is, when the magnitude of the two signals applied to the comparator circuit are equal. Accordingly, resistors 410, 415, and 416 are selected to define a threshold impedance somewhere between the maximum off-hook (low) impedance of a long line and the minimum on-hook (high) impedance of a long -line. Thus, when either a long or a short line presents a low impedance level, the potential applied to transistor 420 is greater than that applied to transistor 421, and the output signal taken across resistor 412 will assume a first level indicative of the off-hook state in conjunction with the comparator circuit. Conversely, when the line presents a high impedance level, the magnitude of the potential applied to transistor 420 is less than that applied to transistor 421. Thus the output signal assumes a second level indicative of the on-hook state.